Start with a load calculation for correct sizing and a complete and thorough evaluation of ductwork system including size, design, and insulation properties.

Where is existing furnace located? What is size and efficiency?

IMO

Thanks Tiger for your response. The furnace is in the basement. It is a Bryant Quietline Deluxe furance model 397EAW036100. I couldn't get the size or any other specifications as its very old and the stickers are torn. Bryant website also doesn't have manuals for this model. Please help..

You failed to answer my question about location of furnace.

The existing furnace model number suggests a 100 KBTU size,, probably no more than 80% efficient. Any idea of its age?

First of all the uneven temperature suggests ductwork problems. This has to be evaluated closely.

Is basement at or below grade? Most homeowners aren't aware that basements below grade have a low BTU load for both heating and cooling.

You need to get a return installed for attic if comfort is important to you. I might even consider a zoning control for this area if practical.

If the basement will be finished into a living area in the future, now s the time to take this into consideration if purchasing new HVAC.

IMO

Hi Tiger..mine is a below grade basement. I did some digging and found that the furnace is a 1976 model. Location wise, I am not sure what you mean. The furnace is located in one of the corners of the basement adjacent to the exterior access to basement and a glass block window. Thanks for all the tips. I will have the zoning control option and duct work system evaluated.

The problem is you have a furnace in the basement trying to heat and cool a 500 sq. ft attic with only two supply vents. You will need more supply vents and probably a return. You should investigate if it is feasible to add more supply vents to the attic and improve the air flow.

Is the roof deck is the ceiling of the attic rooms? Do you know the amount of insulation in the attic ceiling?

The heating and cooling load of the attic is very different than that of the main level and basement. I would make the attic a separate zone with its own thermostat. I would also make the duct work modifications for the basement now if you have firm plans to finish it in the future. The basement should also be a separate zone if you really want good temperature control. If you are willing to live with a basement with is a little colder in the winter then you don't have to do zoning.

Plan on getting a 95%+ furnace preferably with a variable speed blower. The top brand pairs are Carrier/Bryant, Trane/American Standard, Rheem/Rudd. Find a contractor who will do a load calculation and a thorough review of the duct work. Most contractors are not good at zoning, so you have to find one with experience. You don't have to change the AC now, but you should have a plan in place as to how it would be upgraded in the next few years.

What size is AC condenser?

You might consider adding a small supply to attic bathroom. As far as the other supplies to the attic bedrooms, dealer should check the size of the supply runs before adding another. probably undersized. Definitely a return is needed for attic zone.

IMO

@Mike: Thanks so much for your message and analysis. Yes, the roof deck is the ceiling of the attic rooms. I don't know the amount of insulation in the attic ceiling. Is there any guideline on the amount of attic insulation I need?

@Tiger, thanks. I will plan to get more supplies in the attic. But it seems like I can only have the return on the attic stair wall, but not in the attic. will this be efficient?

I have a 2 1/2 ton Trane condensing unit , i have replaced the capacitor, and now when the unit starts the fan runs for a little and the compressor keep going?...why does the fan stop and the compressor keep going?..PLZ HELP!

Farmer, you will have a lot more people see your problem if you start your own thread. Sounds like the motor is dropping out on thermal protection. May be time to replace.

You want a minimum of R30 insulation, preferably more.